Cobalt-chromium base alloy and articles produced therefrom

ABSTRACT

ALLOYS CONTAINING COBALT, CHROMIUM AND CARBON, PARTICULARLY IN A RELATIONSHIP WHEREIN TWO PRIMARY PHASES FREEZE SIMULTANEOUSLY FROM A MELT, ARE CAST INTO ARTICLES HAVING SUPERIOR HIGH TEMPERATURE STRENGTH AND OXIDATION/SULFIDATION RESISTANCE, THE CAST ARTICLES COMPRISING   A MATRIX CONTAINING ABOUT 30 WEIGHT PERCENT CHROMIUM AND A DISPERSED PHASE COMPRISING ABOUT 35-40 PERCENT OF A MIXED CARBIDE OF THE COMPOSITION (CR, CO)23C6.

Jan. 5., 1971 LEMKEY ET AL 3,552,953

COBALT-CHROMIUM BASE ALLOY AND ARTICLES PRODUCED THEREFROM Filed Jan. 10, 1969 5 Sheets-Sheet 1 1971 F. D. LEMKEY ET AL 3,552,95

COBALT- CHROMIUM BASE ALLOY AND ARTICLES PRODUCED THEREFROM Filed Jan. 10, 1969 5 Shee ts-Sheet 2 FIG 3A Fleas Jan. 5, 1971 LEMKEY ET AL 3,552,953

COBALT-CHROMIUM BASE ALLOY AND ARTICLES PRODUCED THEHEFHOM Filed'Jan. 10, 1969 5 SheetsShe0t 5 THERMAL FATIGUE BOW 1 STEADY STATE AT [950F, I2 H/?.S.AT5000P$I,DVA/AMIC CYCLE MAX. Z/00F, I5 SEC. HEATING,3OSEC. COOLING TU 400F IZU mom IN 713C 7 Y Q Y Q 80 M/l/P M 30z I Co-(Cr(5)7 (3' E 60 CFflCK/A C; CQQ Ca 1 )23 06 0 l 1 l 1 l l l l 1' J IZHRSZOO 400 600 800 I000 I200 I400 I600 I800 2000 2200 2300 CYCLES United States Patent 3 552 953 COBALT-CHROMIUlVI BASE ALLOY AND ARTICLES PRODUCED THEREFROM Franklin D. Lemkey, Oxford, England, and Earl R.

Thompson, Glastonbury, Conn., assignors to United Aircraft Corporation, East Hartford, Conn., a corporation of Delaware Filed Jan. 10, 1969, Ser. No. 790,369 Int. Cl. C22c 19/00 US. Cl. 75-171 11 Claims ABSTRACT OF THE DISCLOSURE Alloys containing cobalt, chromium and carbon, particularly in a relationship wherein two primary phases freeze simultaneously from a melt, are cast into articles having superior high temperature strength and oxidation/sulfidation resistance, the cast articles comprising a matrix containing about 30 Weight percent chromium and a dispersed phase comprising about 35-40 percent of a mixed carbide of the composition (Cr, Co) C BACKGROUND OF THE INVENTION The present invention relates in general to the high temperature alloys and articles produced therefrom particularly those having utility in gas turbine engine applications.

The capabilities of the current nickel and cobalt-base superalloys are being severely taxed in the advanced gas turbine engines since they are exposed to high stress levels at temperatures in excess of 85 percent of their melting points. While the performance and endurance of these alloys have been improved by design techniques such as air cooling and by manufacturing advances such as unidirectional solidification, such measures offer only interim solutions to the basic problem.

In a copending application entitled Anisotropic Polyphase Structure of Monovariant Eutectic Composition, Ser. No. 734,821, filed June 5, 1968 by the present inventors and sharing a common assignee with this application, there have been disclosed a number of alloy compositions which respond to unidirectional solidification to provide an aligned microstructure wherein rods or lamellae of one phase are embedded in a matrix of another phase. Included among the described compositions is an alloy system of cobalt, chromium and carbon, particularly at the monovariant eutectic composition of, by Weight, 35-45 percent chromium, 2.6-2.2 percent carbon, balance cobalt. The above-mentioned alloys, like certain eutectics as indicated in the patent to Kraft 3,124,- 452, may be solidified to produce articles of pronounced anisotropy. In the ternary cobalt-chromium-carbon system, as above, a dispersed carbide phase (Cr, Co)-;C aligns in a cobalt-base solid solution matrix.

Based on their strength characteristics and excellent high temperature oxidation/sulfidation resistance, the alloys of the cobalt-chromium-carbon system offer great potential utility in the advanced gas turbine engines. The particular applications and the particular needs in a given application will, of course, vary. As is typically the case with articles of pronounced anisotropy, the strength characteristics of such articles are usually preferential as to direction and, in those articles where increased omnidirectional strength is of more fundamental importance, the anisotropic articles may often be less preferred than components exhibiting a lesser degree of directional orientation.

3,552,953 Patented Jan. 5, 1971 "ice The present invention generally contemplates alloys nominally of the cobalt-chromium-carbon type which substantially solidify according to the eutectic-type reaction wherein two primary phases freeze simultaneously from a multi-component system to form a matrix phase consisting of a cobalt base alloy and a dispersed phase consisting of a mixed carbide of the M C type. The alloys of this invention solidify such that the carbide is randomly dispersed throughout the matrix or, upon unidirectional solidification, is dispersed in a skeletal distribution in the matrix. As such, these alloys display substantial omnidirectional strength.

The unmodified ternary alloys of this invention, in terms of the basic cobalt, chromium, carbon relationship, occupy a limited segment on the eutectic trough which exists in the ternary phase diagram of this system. Accordingly, the ternary alloys lie below the liquidus trough and within the two phase field, solidifying, in fundamental terms, according to the monovariant eutectic reaction:

The ternary alloy composition, by weight, solidifying by this reaction consists essentially of, 45.2-49.2 percent cobalt, 49-53 percent chromium and about 1.8 percent carbon. The particularly preferred ternary alloy is formulated at the composition, by weight, 49.2 percent cobalt, 49 percent chromium, and 1.8 percent carbon.

In general, the present basic alloy is tolerant toward the addition or substitution of certain materials, and, in certain applications some change from the basic ternary composition is desirable. Accordingly, the alloys mentioned herein also include those alloys that substantially solidify according to the reaction L=u+fi where a comprises a cobalt-chromium-base alloy containing up to about, by weight, 10 percent nickel, 5 percent iron and 2 percent aluminum, yttrium and the rare earth elements, and 5 consists of a dispersed carbide of the M C type, or of the generic type (Cr, Co) C where M includes both cobalt and chromium, together with the other stable carbide-forming elements of the a phase. These alloys, in terms of the melt composition, may be generated from a formulation comprising by weight, 45-55 percent chromium, 1.7-2.2 percent carbon, up to 10 percent nickel, up to 5 percent iron, up to 2 percent aluminum, yttrium and the rare earth elements, balance essentially cobalt.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a photomicrograph of a unidirectionally solidified specimen of the ternary alloy at the preferred composition taken transverse to the direction of growth X 500).

FIG. 2 is a photomicrograph of the same specimen taken in longitudinal section (X500). 5

FIG. 3A is a space model phase diagram illustrating a eutectic trough, e-e in a ternary system.

FIG. 3B is an isopleth taken along the dotted lines of FIG. 3A.

FIG. 3C is a projection of the eutectic trough and the solubility curves on the basal triangle.

FIG. 4 is the liquidus diagram for the cobalt-chromiumcarbon system.

FIG. 5 is a graph demonstrating the tensile strength of the ternary alloy as a function of temperature.

FIG. 6 is a graph depicting the stress-rupture life of the alloys of this invention as compared to a number of competitive alloys.

FIG. 7 is a chart demonstrating the results of vane cyclic sulfidation-erosion testing of these alloys as a function of time.

FIG. 8 is a graph illustrating the extent of specimen bowing in thermal fatigue testing of various alloys, the arrows indicating the onset of surface cracking.

DESCRIPTION OF THE PREFERRED EMBODIMENTS As hereinafter utilized for the sake of brevity and as set forth in the drawings the shorthand expression Co-(Cr, Co) C is utilized to generally indicate the alloys of the present invention.

The basic mechanism involved in the solidification of the present alloys is best explained by first referring to the basic ternary system. Solidification of the ternary alloys proceeds according to the monovariant reaction. This reaction will be understood to have reference to those systems wherein n-l phases solidify simultaneously from the liquid of an n component system. This is to be distinguished from the invariant eutectic reaction which refers to the isothermal and simultaneous crystallization of n phases in an n component system.

Accordingly, in terms of the ternary alloys, the present approach contemplates the solidification of a melt whose composition substantially corresponds to that at the eutectic trough of some phase diagrams, such a trough being illustrated by way of example by the line e-e of the model ternary diagram of FIG. 3A, which shows a three phase region passing continuously from one binary eutectic horizontal to the other along the line 2-2 The vertical section or isopleth indicated by the dotted lines of FIG. 3A is reproduced as FIG. 3B. A liquid of composition X will solidify over the temperature range T to T2- The equilibrium solidification of composition X may be conveniently described by referring to FIG. 3C which is a projection of the eutectic trough and the solubility curves on the basal triangle. Also shown are the tie triangles whose vertices indicate the composition of the liquid and two solid phases in equilibrium. At each temperature level there is a tie triangle. These triangles reduce to the eutectic horizontal in the terminal binary phase diagrams. For the solidification of composition X only the tie triangles corresponding to the beginning of freezing, T and the termination of freezing, T have been shown in FIG. 3C. There are in fact a continuous series of tie triangles between these temperatures. During freezing, adjustments of the liquid and the two solid compositions are made along the eutectic trough and solubility curves, respectively, until at T the last liquid to freeze is of composition L and the solid phase compositions have shifted from T1OL1 to T u and from T a to T a With reference to FIG. 3A, solidification of a melt whose composition lies along the trough ee will result in the two phase separation of ea l-L from the liquid. The composition of the 0:; phase may thus be varied from a to a, at the eutectic temperature, and from c to 0 at room temperature. The composition of the conjugate 1x phase, on the other hand, varies in dependence upon the composition of the a; phase. The amount of each phase is variable and can be calculated on a tie line utilizing the lever rule.

While the term monovariant eutectic reaction may conveniently be utilized to describe the above-mentioned solidification reaction in the ternary system including the basic ternary alloys of this invention, the term is not always applicable to the higher order systems. The basic reaction desired here involves the simultaneous solidification of only two primary phases from a melt regardless of the number of components in the system. Accordingly, this solidification mechanism should be distinguished, not only from the eutectic reaction of the type L=0c+}8+'y, but also from the higher order monovariant eutectic reactions where 3 or more phases freeze simultaneously from a melt.

As previously mentioned, solidification of the alloys of this invention proceeds according to the reaction expressed as L=a+fl. Expressed nominally in different terms, this reaction with the alloys of this invention may be seen to proceed substantially according to the reaction:

Loosely speaking, it may be characterized as a eutectictype reaction. This reaction is to be distinguished from that described in the copending application previously mentioned wherein solidification occurs according to the reaction: L='y +M-;C to form an aligned fibrous carbide phase dispersed in a cobalt alloy matrix.

In the present case two basic distinctions may be made. Even in the case of the ternary alloy at the monovariant eutectic composition, the present formulation does not result in the aligned fibrous carbide phase of the prior compositions but rather in a randomly dispersed mixed carbide phase or, in the case of the unidirectionally solidified structure, in a skeletal distribution of the carbide. As a result, the present castings exhibit a greater degree of isotropy and omnidirectional strength than the aligned structures including the M- C carbide.

The fundamental premise of the present invention, therefore, resides in the formation and random dispersion of the M C -type carbides in a cobalt base alloy matrix of high chromium content. In directionally solidified form these carbides will normally be distributed in a skeletal structure.

The basic alloy system from which these structures are formed is the ternary alloy of cobalt, chromium and carbon solidifying acording to the monovariant eutectic reaction, particularly in the compositional range, by weight, of 45.2-49.2 percent cobalt, 49-53 percent chromium, and about 1.8 percent carbon.

A ternary composition consisting of 49.2 percent cobalt, 49 percent chromium and 1.8 percent carbon freezes to form castings consisting of a matrix phase of cobalt containing about 30 weight percent carbon in solid solution and about 35-40 percent of a dispersed carbide phase (Cr, Co) C with a minimum freezing temperature of about 2370 F. Its elastic modulus at room temperature is 40x10 p.s.i. which compares favorably with the reported elastic moduli of the commercial cobalt-base superalloys in the range of 3.0-36 10 p.s.i. In terms of density, the present alloy density of 7.91 g./cc. is an improvement over the densities of the commercial superalloys in the general range of 8.8-9.2 g./cc. The specific properties of these alloys in terms of their comparative strengths, bow resistance and oxidation/sulfidation resistance are set forth in FIGS. 5-8.

The basic ternary composition exhibits a reasonable tolerance insofar as modifications to the basic alloy chemlstry are concerned. Depending on a number of factors, but particularly the use to which the alloys are to be put, it is not only possible but also, in many cases, desirable to make limited substitutions in or additions to the basic ternary composition. For example, articles formed of the basic ternary composition unidirectionally solidified at about 10 cm./hr. have displayed unacceptable creep-rupture properties in the intermediate temperature range of 1500-1600 F. While the reasons for this behavior are not fully understood, the behavior is believed to be associated with the allotropic transformation of the cobaltchromium solid solution. In any event, for use in those gas turbine engines operating in the intermediate temperature regimes, modification of the alloyis suggested. Accordingly, the basic alloy was modified, in some instances, to include nickel, a face-centered cubic stabilizer. Nickel substitutions of up to about 10 weight percent are readily tolerated and serve to reduce the transformation temperature and thus to shift it out of the critical range.

In further investigations it was discovered that the transformation temperature is similarly and more efiiciently reduced by substituting iron for a portion of the cobalt in these alloys. Similarly, additions of aluminum, yttrium and therare earth'elements may advantageously be added to these alloys. Such additions are primarily included as modifiers of the solid solution matrix, aluminum, yttrium and the rare earth elements, such as lanthanum frequently being added to the superalloys to promote their oxidation/erosion resistance.

A number of tests have been conducted on these alloys both in the modified and unmodified condition. The results of a number of these tests are set forth in Tables I and II.

bodiments, these are illustrative only. It will be understood that the invention is not to be limited to the exact details described, for obvious modifications will occur to those skilled in the art. 1

What we claim and desire to secure by Letters Patent of the United States is:

1. An alloy substantially solidifying according to the eutectic-type reaction wherein two primary phases freeze simultaneously from a multi-component system, one of the primary phases comprising a matrix phase consisting of a cobalt-chromium-base alloy, the other primary phase TABLE I [Longitudinal tensile properties of (Co)(r, C0)2306] Percent Nominal Tensile Modulus 1 of strain strain Temp., strength elasticity at rate (in./ F. (p.s.i.) (p.s.i) failure in./min.) Comments 70 177,000 39.2X10 0.94 .01 70 173, 000 38. 7X10 0. 98 01 H.'1.12)5 hrs. at 2,200 F. air

coo e 70 172, 000 46. X10 0. 96 .01 H.T. 100 hrs. at 2,200 F.

11 0 quenched. 70 40. 65;. 8X10 01 Static long modulus.

34. Bil. 1X10 01 Static 45 modulus. 40. 0=l=1. 2X10 81 Static transverse modulus.

7 01 .005 Longitudinal. .005 45. 005 Transverse.

. 01 .07 As cast. .07 H. 205 hrs. at 2,200 F. air

01 cooled.

1 Dynamic modulus 40.5-42.0X p.s.i. at room temperature, 31.2-32.5X10 p.s.i. at

comprising a dispersed phase consisting of a mixed car- Elonbide of the generic type (Cr, Co) C TABLE II Temp., Stress, Hours Hours Hours to Percent gation F. k.s.i. to 0.5% to 1.0% rupture prior final Creep rupture results for (Co)(Cr, C0)23Cd, longitudinal direction 25. 0 -10. 0 -14. 0 26. 4 20. 0

2,000 (trans.)

What has been described herein is an alloy system wherein the alloys substantially solidify according to the eutectic-type reaction wherein two primary phases freeze simultaneously from the melt, one of the primary phases comprising a matrix phase consisting of a cobalt-chrornium-base alloy, the other primary phase comprising a dispersed carbide phase of the M C type. Those skilled in the art will recognize that the described compositions may be varied to some extent while substantially maintaining the basic solidification mechanism described. Accordingly, while the invention has been described in detail with reference to certain examples and preferred em- 2. An alloy according to claim 1 wherein the matrix phase consists of a cobalt-base alloy containing about 25-35 weight percent chromium in solid solution.

3. An alloy according to claim 1 wherein the matrix phase consists of a cobalt-base alloy containing about 30 weight percent chromium in solid solution.

4. An alloy substantially solidifying according to the reaction L=a+;8 where on comprises an alloy consisting essentially of, by weight, 25-35 percent chromium, up to about 10 percent nickel, up to about 5 percent iron and up to about 2 percent aluminum, yttrium and the rare earth elements and where [3 consists of a dispersed carbide of the M C type where M includes cobalt, chromium and the other stable carbide-forming elements of the a phase.

5. An alloy according to claim 4 wherein the an alloy contains about 30 percent chromium.

6. A unidirectionally solidified metallic article having a substantially two phase microstructure consisting of a. matrix phase and a dispersed phase, the matrix phase consisting essentially of a cobalt-chromium-base alloy, the dispersed phase consisting essentially of a mixed carbide of the generic type (Cr, Co) C in a substantially skeletal distribution.

7. A unidirectionally solidified metallic article having a substantially two phase microstructure consisting of a matrix phase and a dispersed phase, the matrix phase consisting essentially of a cobalt-base alloy containing, by weight, 25-35 percent chromium, up to about 10 percent nickel, up to about 5 percent iron, and up to about 2 percent aluminum, yttrium and the rare earth elements, the dispersed phase consisting of a mixed carbide of the C C type Where M includes cobalt, chromium and the other stable carbide forming elements of the matrix phase alloy.

8. An alloy consisting essentially of, by weight, 45-55 percent chromium, 1.7-2.2 percent carbon, up to 10 percent nickel, up to 5 percent iron and up to 2 percent aluminum, yttrium and the rare earth elements, balance essentially cobalt, said alloy having a microstructure of a dispersion of the M C type carbides in a cobalt base alloy matrix of high chromium content. I References Cited 9. Analloy according to claim wherein the chromium UNITED STATES PATENTS content is about 49 percent by Weight and the carbon cony 5 tent is about 1.8 percent by weight. 3124452 3/1964 Kraft 75"134 10. An alloy consisting of, by weight, 49-53 percent 3434827 3/1969 'Lemkey 75' 134 romium, 1.8-2 percent carbon, balance essentially 5 3,434,892 3/ 19 69 Heimke 7 5 134 balt, said alloy having a microstructure of a dispersion of the M C type carbides in a cobalt base alloy matrix of RICHARD N Pnmary Exammer high chromium content. v y Y 11. A eutectic alloy consisting essentially of, by Weight, 10 X- about 49.2 percent cobalt, 49 percent chromium, and 1.8 135, percent carbon.

32 3 UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No, D t d J8I1'L13I'Y 5,

Inventor(s) Franklin D. Lemkey and Earl R. Thompson It is certified that error appears in the above-identified patent and that said Letters Patent are hereby corrected as shown below:

Column 6, line 67, "C 0 should read -M C Column 7, line 8, "M 0 should read --M C Signed and sealed this 6th day of April 1971.

(SEAL) Attest:

EDWARD M.FLETCHER,JR. WILLIAM E. SCHUYLER, JR. Attesting Officer Commissioner of Patents 

